It is known to electrolytically polish a workpiece which has been previously machined by EDM, the electrolytical polishing being effected by means of electrode tool of the same shape as the electrode tool used in the course of the EDM machining. Such a process, which is described in U.S. Pat. No. 3,689,387 uses, for polishing the inner surface of a cavity in a workpiece, a cathode of reduced over-all dimensions relative to the electrode tool used for machining by EDM the cavity in the workpiece. The reduction in size of the cathode, relative to the electrode tool used in EDM, is necessary for obtaining a constant gap of sufficient width between the tool surface and the areas of the workpiece which are polished.
It is evident that if the areas of the workpiece which are to be polished are on the outside of the workpiece, the cathode is oversize relative to the electrode tool used during the EDM machining. In order to achieve a good quality and evenness of polishing, the distance between the cathode and the surface of the workpiece must be ten times as wide as the gap during machining of the workpiece by electrical discharges, which results in requiring a cathode of appropriate dimensions for the polishing operation. Making a second tool of a size different from that of the tool used for machining the workpiece by EDM is one of the important disadvantages of such a machining process.
The present invention has for principal object to overcome such disadvantages by permitting to use the same tool, or eventually a tool of the same dimensions, to effectuate the polishing step, insuring that an adequate and constant distance is maintained between the cathode and the workpiece.